Communication devices such as wireless devices are also known as e.g. user equipments (UE), mobile terminals, wireless terminals, and/or mobile stations. Wireless devices are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, tablet computers or surf plates with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless device or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a radio network node. A cell is the geographical area where radio coverage is provided by the radio network node.
The radio network node may e.g. be a base station such as a Radio Base Station (RBS), eNB, eNodeB, NodeB, B node, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size.
Further, each radio network node may support one or several communication technologies. The radio network nodes communicate over the air interface operating on radio frequencies with the wireless terminals within range of the radio network node. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. Data transmission in LTE is controlled by the radio base station.
Upon demand on higher bandwidth and higher data rate applications, LTE-Advanced as of 3GPP Release 10 introduces Carrier Aggregation (CA).
Carrier Aggregation allows expansion of effective bandwidth delivered to a wireless device through concurrent utilization of radio resources across multiple carriers. This means that several Component Carriers (CCs) may be aggregated to form a larger overall transmission bandwidth. A wireless device that is CA capable may be configured with multiple component carriers, corresponding to multiple serving cells, from a radio network node. Then, the wireless device with reception and/or transmission capabilities for carrier aggregation may simultaneously receive and/or transmit on these multiple component carriers originating from the same radio network node. Carrier Aggregation is supported for both contiguous and non-contiguous component carriers.
It is possible to configure a wireless device to aggregate a different number of component carriers originating from the same radio network node and of possibly different bandwidths in the UL and the DL. The number of DL component carriers that may be configured depends on the DL aggregation capability of the wireless device and of the aggregation capability of the radio network node. The number of UL component carriers that may be configured depends on the UL aggregation capability of the wireless device and of the aggregation capability of the radio network node. Component carriers originating from the same radio network node need not provide the same coverage.
As mentioned above, each component carrier corresponds to a serving cell. Thus, the wireless device may have multiple serving cells, each serving cell operating on a respective component carrier.
The wireless device may connect to a primary serving cell, also referred to as a primary cell or PCell. The PCell is served by a Primary Component Carrier (PCC), originating from a serving radio network node, such as an eNB. Additionally, the wireless device may also connect to one or several secondary serving cells also referred to as secondary cells or SCells. Each SCell is served by a corresponding Secondary Component Carrier (SCC), originating from the radio network node. The PCC may be regarded as the anchor carrier for the wireless device and is thus used for basic functionalities such as radio link failure monitoring. The Radio Resource Control (RRC) connection is handled by the PCell, served by the PCC. After connection establishment, one or several SCells may be configured and activated, to provide additional radio resources.
Via RRC signaling, the wireless device may indicate to a serving radio network node its capability to support one or more SCells in the downlink, as well as one or more SCells in the uplink. The serving radio network node may alternatively receive information from another network node about the capability of the wireless device to support one or more SCells.
A wireless device camps on one cell at the time. When going from an idle to a connected mode, the wireless device attaches to the cell which the wireless device is currently camping on. The cell to which the wireless device successfully attaches becomes the PCell of this wireless device. The radio network node from which the PCell originates may then configure the wireless device with one or more SCells, if the wireless device is capable of supporting one or more SCells in the downlink, and/or one or more SCells in the uplink.
In some scenarios, there are several cells available for the radio network node to use as SCells for the wireless device. In existing solutions, the radio network node will in these cases have to either randomly select one or several suitable SCells or perform measurements to find one or several suitable SCells. One main criterion for the wireless device to be able to use a cell as an SCell is that the coverage of that cell is sufficient.
If the radio network node randomly selects an SCell which is not suitable for the wireless device, the selection procedure will have to continue or be restarted. This leads to an increased time from the moment when the wireless device has attached to the PCell until the moment when it is configured and activated with one or several suitable SCells and can benefit from carrier aggregation, as compared to a scenario in which a suitable SCell is selected at once. Measurements are also time consuming and lead to an increased time from the moment when the wireless device has attached to the PCell until the moment when it is configured and activated with one or several suitable SCells and can benefit from carrier aggregation.
Measurements as well as incorrect random selections also add additional reconfiguration efforts between the radio network node and the wireless device. This reduces wireless device's performance and the network capacity, due to extra signaling.